When I first tried out the test-driven development (it was around 1998, I think), I was fascinated how it helped me to design better APIs. My unit tests were the first clients of my code, so that my classes obtained a logical and easy-to-use interface, quite automatically.

Some time later I’ve realized that if you have a lot of unit tests, they can detect regressions and therefore support you during refactoring. I’ve implemented two projects, each took a couple of years, and have written around 200 unit tests for each.

And then I’ve stopped writing unit tests in such big counts. My unit tests have really detected some regressions from time to time. That is, around 5 times a year. But the efforts writing and maintaining them were much higher than any advantages of having detected a regression before manual testing.

But still, I was missing the first advantage of TDD, the logical and easy-to-use interfaces. So I’ve started to do Client Driven Development.

The problem with the unit tests is that they don’t have any direct business value per se. They might be helpful for business goals, but in a very indirect way. I’ve replaced them with a client code that does have some direct business value.

For example, I’m developing a RESTful web service. I roughly know what kind of queries and responses it must support. I start with developing a HTML page. In there, I write an <a> tag with all the proper parameters to the web service. I then might write some text around it, documenting the parameters of the service. Then I open this page in the browser and click on the link, which predictably gives me a 404 error, because the web service is not yet implemented. I then proceed with implementing it, reloading my page and generally using it in place of a unit test.

Of course, this approach has the drawback that, unlike in a unit test, I don’t check the returning values and thus this page cannot be run automatically. If you want, you still can replace this link with an AJAX call and check the returning values – I personally don’t believe that these efforts would pay off at the end of the day. More important is that this page has an immediate business value. You can use it as a rough and unpolished documentation for your web service. You can send it to your customer, or another team writing some client, etc.

If the web service is designed in a way that it is hard to get away with <a> and <form> tags, I would write some JavaScript or Silverlight code to call it properly. In this case, the page might have more business-relevant functions. For example, when it loads, it might request and display some data from the web service, in a sortable and scrollable grid, and allow you to edit them, providing you with a very low-level “admin” interface to the service.

This approach is not constrained by web development. I’ve used it for example for inter-process communication, and if my code has not yet been refactored out, it is flying now in passenger airplanes, and running inside of TV sets in many living rooms. In this variant, I start developing the inter-process communication by creating a bash script or a trivial console app that would send the messages to another process. I implement corresponding command-line options for them. When I’m ready, I start developing the receiving part, inside of some running process. This has the similar effect on the API design as unit testing, but has the advantage that you can use it during debugging, or even in production, for example in some startup scripts.

I’m not an inventor of this approach, indeed I often see this approach in many open source projects, but I’m not aware of any official name for it.